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Abstract:

A removable water-dispersible acrylic pressure-sensitive adhesive
composition according to the present invention includes an acrylic
emulsion polymer (A), a water-insoluble crosslinking agent (B), and a
carboxylic-acid-containing acrylic polymer (C). The polymer (A) includes
a (meth) acrylic alkyl ester and a carboxyl-containing unsaturated
monomer as essential constitutive monomers and has a content of the
(meth) acrylic alkyl ester of 70 to 99.5 percent by weight and a content
of the carboxyl-containing unsaturated monomer of 0.5 to 10 percent by
weight, each based on the total amount of constitutive monomers. The
polymer (C) includes (meth) acrylic acid as an essential constitutive
monomer and has a total content of acrylic acid and methacrylic acid of
11 percent by weight or more based on the total amount of constitutive
monomers.

Claims:

1. A removable water-dispersible acrylic pressure-sensitive adhesive
composition comprising: an acrylic emulsion polymer (A) including a
(meth)acrylic alkyl ester and a carboxyl-containing unsaturated monomer
as essential constitutive monomers and having a content of the
(meth)acrylic alkyl ester of from 70 to 99.5 percent by weight and a
content of the carboxyl-containing unsaturated monomer of from 0.5 to 10
percent by weight, each based on the total amount of constitutive
monomers; a water-insoluble crosslinking agent (B); and a
carboxylic-acid-containing acrylic polymer (C) including (meth)acrylic
acid as an essential constitutive monomer and having a total content of
acrylic acid and methacrylic acid of 11 percent by weight or more based
on the total amount of constitutive monomers.

2. The removable water-dispersible acrylic pressure-sensitive adhesive
composition according to claim 1, wherein the acrylic emulsion polymer
(A) is a polymer polymerized with a reactive emulsifier including a
radically polymerizable functional group in a molecule.

3. The removable water-dispersible acrylic pressure-sensitive adhesive
composition according to claim 1, wherein the water-insoluble
crosslinking agent (B) has a functional group reactive with carboxyl
group, and wherein the functional group is contained in a number of moles
of from 0.3 to 1.3 moles per 1 mole of carboxyl group in the
carboxyl-containing unsaturated monomer used as the constitutive monomer
of the acrylic emulsion polymer (A).

4. A pressure-sensitive adhesive sheet comprising: a substrate; and a
pressure-sensitive adhesive layer arranged on at least one side of the
substrate and formed from the removable water-dispersible acrylic
pressure-sensitive adhesive composition of claim 1.

5. The pressure-sensitive adhesive sheet according to claim 4, wherein
the pressure-sensitive adhesive layer has a solvent-insoluble content of
90 percent by weight or more and an elongation at break of 200% or less.

6. The pressure-sensitive adhesive sheet according to claim 4, as a
surface-protective film for an optical element.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a removable, water-dispersible
acrylic pressure-sensitive adhesive composition. Specifically, the
present invention relates to a removable water-dispersible acrylic
pressure-sensitive adhesive composition capable of forming a
pressure-sensitive adhesive layer which less suffers from appearance
defects (poor appearance) such as dimples, thereby has a good appearance,
is satisfactorily resistant to increase in adhesive strength with time,
and less causes staining. The present invention also relates to a
pressure-sensitive adhesive sheet including a pressure-sensitive adhesive
layer formed from the pressure-sensitive adhesive composition.

BACKGROUND ART

[0002] In production and working processes of optical elements (optical
members; optical materials) typified by optical films such as deflector
plates, retardation. films, and anti-reflective films, surface-protecting
films are used as to be affixed to surfaces of such optical elements for
the purpose typically of preventing surface defects and soiling,
improving cutting workability, and suppressing cracking (see Patent
Literature (PTL) 1 and 2). As such surface-protecting films, there are
generally used removable pressure-sensitive adhesive sheets each
including a plastic film. substrate and, arranged on a surface thereof, a
removable pressure-sensitive adhesive layer.

[0003] Solvent acrylic pressure-sensitive adhesives have been generally
used as pressure-sensitive adhesives for these surface-protecting films
(see PTL 1 and 2). These solvent acrylic pressure-sensitive adhesives
contain organic solvents and are thereby intended to be witched to
water-dispersible acrylic pressure-sensitive adhesives from the viewpoint
of working environmental conditions upon coating (see PTL 3 to 5).

[0004] The surface-protecting films are desired to exhibit sufficient
adhesiveness during affixation to optical elements. In addition, they are
desired to have satisfactory peelability (removability) because they are
peeled off (removed.) after usage typically in production processes of
optical elements. Fox having satisfactory removability, such
surface-protecting films should have small peel strength (light
releasability) and, in addition, properties not to be increased in
adhesive strength (peel strength) with time after the affixation to an
adherend such as an optical element (prevention of increase in adhesive
strength).

[0005] For obtaining the properties such as light releasability (good
removability) and prevention of increase in adhesive strength, it is
effective to use a water-insoluble crosslinking agent in a
pressure-sensitive adhesive pressure-sensitive adhesive composition) As
examples of a pressure-sensitive adhesive composition. using a
water-insoluble crosslinking agent, there are known. removable
water-dispersible acrylic pressure-sensitive adhesive compositions each
containing an oil-soluble crosslinking agent (see PTL 6 and 7).

[0006] However, the aforementioned pressure-sensitive adhesive
compositions and other water-dispersible acrylic pressure-sensitive
adhesive compositions each using a water-insoluble crosslinking agent
include large particles of the water-insoluble crosslinking agent that
has been insufficiently dispersed and remained, and this often causes
"dimples" and other appearance defects of the surface of a resulting
pressure-sensitive adhesive layer upon formation thereof. For these
reasons, the inspection of an adherend while a surface-protecting film is
affixed thereon may be impeded or other problems may occur particularly
when such a water-insoluble crosslinking agent is used for a
pressure-sensitive adhesive layer of the surface-protecting film,

[0007] Accordingly, under current circumstances, there has not yet been
obtained a removable water-dispersible acrylic pressure-sensitive
adhesive composition capable of forming a pressure-sensitive adhesive
layer which excels in adhesiveness and removability (particularly in
prevention of increase in adhesive strength), less suffers from
appearance defects such as "dimples", and has a good appearance.

[0008] Typically in a surface-protecting film use (particularly in a use
of surface-protecting film for an optical element), staining
(contamination) of the adherend surface causes problems such as adverse
effects on optical properties of the optical element. The staining is due
typically to remaining of the pressure-sensitive adhesive on the surface
of the adherend (e.g., optical element) (so-called "adhesive residue")
and due to transfer of components contained in the pressure-sensitive
adhesive layer to the adherend surface upon removal of the
pressure-sensitive adhesive sheet. To avoid these, pressure-sensitive
adhesives and pressure-sensitive adhesive layers are strongly desired to
less stain the adherend.

[0016] Accordingly, an object of the present invention is to provide a
water-dispersible acrylic pressure-sensitive adhesive composition capable
of forming a pressure-sensitive adhesive layer which is a removable
pressure-sensitive adhesive layer, has a good appearance (less suffers
from dimples and other appearance defects), is resistant to increase in
adhesive strength, and less causes staining. Another object of the
present invention. is to provide a pressure-sensitive adhesive sheet
having a pressure-sensitive adhesive layer formed from the
pressure-sensitive adhesive composition.

Solution to Problem

[0017] After intensive investigations to achieve the objects, the present
inventors have found that, by including, as constituents, an acrylic
emulsion polymer obtained from constitutive monomers in a specific
composition; a water-insoluble crosslinking agent; and a
carboxylic-acid-containing acrylic polymer obtained from constitutive
monomers in a specific composition, there is obtained a water-dispersible
acrylic pressure-sensitive adhesive composition (removable
water-dispersible acrylic pressure-sensitive adhesive composition)
capable of forming a pressure-sensitive adhesive layer which has a good
appearance, is resistant to increase in adhesive strength, less causes
staining, and is removable. The present invention has been made based on
these findings.

[0018] Specifically, the present invention provides, in an aspect, a
removable water-dispersible acrylic pressure-sensitive adhesive
composition which includes an acrylic emulsion polymer (A), a
water-insoluble crosslinking agent (B), and a carboxylic-acid-containing
acrylic polymer (C). The acrylic emulsion polymer (A) includes a (meth)
acrylic alkyl ester and a carboxyl-containing unsaturated monomer as
essential constitutive monomers and has a content of the (meth)acrylic
alkyl ester of from 70 to 99.5 percent by weight and a content. of the
carboxyl-containing unsaturated monomer of from 0.5 to 10 percent by
weight, each based on the total amount of constitutive monomers. The
carboxylic-acid-containing acrylic polymer (C) includes (meth)acrylic
acid as an essential constitutive monomer and has a total content of
acrylic acid and methacrylic acid of 11 percent by weight or more based
on the total amount of constitutive monomers.

[0019] In the removable water-dispersible acrylic pressure-sensitive
adhesive composition, the acrylic emulsion polymer (A) is preferably a
polymer polymerized with a reactive emulsifier including a radically
polymerizable functional group in a molecule.

[0020] In a preferred embodiment of the removable water-dispersible
acrylic pressure-sensitive adhesive composition, the water-insoluble
crosslinking agent (B) has a functional group reactive with carboxyl
group, and the functional group is contained in a number of moles of from
0.3 to 1.3 moles per 1 mole of carboxyl group in the carboxyl-containing
unsaturated monomer used as the constitutive monomer of the acrylic
emulsion poly=(A).

[0021] The present invention provides, in another aspect,
pressure-sensitive adhesive sheet which includes a substrate; and a
pressure-sensitive adhesive layer arranged on at least one side of the
substrate and formed from the removable water-dispersible acrylic
pressure-sensitive adhesive composition.

[0022] In the pressure-sensitive adhesive sheet, the pressure-sensitive
adhesive layer preferably has a solvent-insoluble content of 90 percent
by weight or more and an elongation at break of 200% or less.

[0023] The pressure-sensitive adhesive sheet may be used as a
surface-protecting film. for an optical element.

Advantageous Effects of Invention

[0024] The removable water-dispersible acrylic pressure-sensitive adhesive
composition according to the present invention is water-dispersible and
has the above configuration. Accordingly, a pressure-sensitive adhesive
layer formed from the pressure-sensitive adhesive composition, and a
pressure-sensitive adhesive sheet having the pressure-sensitive adhesive
layer have satisfactory removability, high adhesiveness, and good
appearances (good appearance characteristics), less cause staining of the
adherend surface after removal thereof, and have satisfactory
less-staining properties. They are resistant to increase in adhesive
strength to the adherend with time and have satisfactory removability.
They are therefore useful for protecting the surfaces of optical films

[0025] These and other objects, features, and advantages of the present
invention will be more fully understood from the following description of
embodiments. All numbers are herein assumed to be modified by the term
"about,"

DESCRIPTION OF EMBODIMENTS

[0026] A removable water-dispersible acrylic pressure-sensitive adhesive
composition according to an embodiment of the present invention
(hereinafter also simply referred to as a "pressure-sensitive adhesive
composition of the present invention") contains an acrylic emulsion
polymer (A), a water-insoluble crosslinking agent (13), and a
carboxylic-acid-containing acrylic polymer (C) as essential components.

[Acrylic Emulsion Polymer (A)]

[0027] The acrylic emulsion polymer (A) in the pressure-sensitive adhesive
composition of the present invention is a polymer including a
(meth)acrylic alkyl ester and a carboxyl-containing unsaturated monomer
as essential constitutive monomers (feed monomer components)
Specifically, the acrylic emulsion polymer (A) is a polymer obtained from
a monomer mixture including a (meth)acrylic alkyl ester and a
carboxyl-containing unsaturated monomer as essential components. Each of
different acrylic emulsion polymers (A) may be used alone or in
combination. As used herein the term "(meth)acrylic" refers to "acrylic"
and/or "methacrylic" (either "acrylic" or "methacrylic", or both).

[0028] The (meth)acrylic alkyl ester is used as a principal monomer
component constituting the acrylic emulsion polymer (A) and mainly plays
the function of developing basic properties as a pressure-sensitive
adhesive (or pressure-sensitive adhesive layer), such as adhesiveness and
peelability (removability). Among such (meth) acrylic alkyl esters,
acrylic alkyl esters are liable to effectively impart flexibility to the
polymer constituting the pressure-sensitive adhesive layer and to thereby
exhibit the effect of allowing the pressure-sensitive adhesive layer to
develop adhesion and tackiness; whereas methacrylic alkyl esters are
liable to impart hardness (rigidity) to the polymer constituting the
pressure-sensitive adhesive layer and to exhibit the effects of
controlling the removability of the pressure-sensitive adhesive layer.
Examples of the (meth)acrylic alkyl esters include, but are not limited
to, (meth)acrylic alkyl esters each having a linear, branched-chain or
cyclic alkyl group preferably having 1 to it; carbon atoms, more
preferably having 2 to 10 carbon atoms, and furthermore preferably having
4 to 8 carbon atoms.

[0030] Of methacrylic alkyl esters, though not limited, methacrylic alkyl
esters each having an alkyl group containing 2 to 16 (more preferably 2
to 8) carbon atoms are preferred, and examples thereof include
methacrylic alkyl esters each having a linear or branched chain alkyl
group, such as ethyl methacrylate, propyl methacrylate, isopropyl
methacrylate, n-butyl methacrylate, isobutyl methacrylate, s-butyl
methacrylate, and t-butyl methacrylate; and alicyclic methacrylic alkyl
esters such as cyclohemyl methacrylate, bornyl methacrylate, and
isobornyl methacrylate. Among them, n-butyl methacrylate is preferred.

[0031] As the constitutive monomers of the acrylic emulsion polymer (A),
methyl methacrylate and/or isohornyl acrylate may he used for the purpose
of improving the appearance of the pressure-sensitive adhesive layer
described later.

[0032] The (meth)acrylic alkyl ester(s) may be suitably selected according
typically to the target tackiness, and each of different (meth) acrylic
alkyl esters may be used alone or in combination.

[0033] The acrylic emulsion polymer (A) contains the (meth) acrylic alkyl
ester (s) in a content of typically from 70 to 99.5 percent by weight,
more preferably from 85 to 99 percent by weight, and furthermore
preferably from 91 to 98 percent by weight, based on total amount of
constitutive monomers (total constitutive monomers) constituting the
acrylic emulsion polymer (A) (as 100 percent by weight). The (meth)
acrylic alkyl ester (s), if present in a content of more than 995 percent
by weight may cause the pressure-sensitive adhesive composition to give a
pressure-sensitive adhesive layer having not so satisfactory anchoring
capability, causing not so little staining, and having not so
satisfactory emulsion stability, This is because the relative content of
the carboxyl-containing unsaturated monomer in the constitutive monomers
becomes low. In contrast, the (meth)acrylic alkyl ester(s), if present in
a content of less than 70 percent by weight, may cause the
pressure-sensitive adhesive layer to have insufficient adhesiveness and
removability. Though not critical, acrylic alkyl esters and methacrylic
alkyl esters may be contained in the (meth) acrylic alkyl esters in a
ratio in content (ratio of the content of acrylic alkyl esters to the
content of methacrylic alkyl esters; [content of acrylic alkyl esters :
content of methacrylic alkyl esters]) of preferably from 100:0 to 30:70
(by weight), and more preferably from 100:0 to 50:50.

[0034] The carboxyl-containing unsaturated monomer can exhibit the
function of forming a protective layer on the surface of emulsion
particles composed of the acrylic emulsion polymer (A) to thereby prevent
shear fracture of the emulsion particles. This function is further
improved by neutralizing the carboxyl group with a base. The stability of
the emulsion particles against shear fracture is more generally referred
to as mechanical stability. One or more multifunctional compounds (e.g.,
multifunctional epoxy compounds) being reactive with carboxyl group may
be used in combination with the carboxyl-containing unsaturated. monomer.
The multifunctional compounds may act as crosslinking points during the
formation of a pressure-sensitive adhesive layer through removal of
water. In addition, the carboxyl-containing unsaturated. monomer may make
the pressure-sensitive adhesive layer to have increased adhesion
(anchoring capability) to the substrate through the multifunctional
compounds. Examples of such carboxyl-containing unsaturated monomers
include (meth) acrylic acid (acrylic acid, methacrylic acid), itaconic
acid, maleic acid, fumaric acid, crotonic acid, carboxyethyl acrylate,
and carboxypentyl acrylate. As used herein the "carboxyl-containing
unsaturated monomer(s)" also include acid anhydride-containing
unsaturated monomers such as maleic anhydride and itaconic anhydride.
Among them, acrylic acid is preferred. because of being liable to have a
high relative concentration, in the surface of emulsion particles and to
thereby form a denser protective layer.

[0035] The content of the carboxyl-containing unsaturated monomer(s) is
from 0.5 to 10 percent by weight, preferably from 1 to 5 percent by
weight, and more preferably from 2 to 4 percent by weight, based on total
amount of constitutive monomers (total constitutive monomers)
constituting the acrylic emulsion polymer (A) (as 100 percent by weight).
The carboxyl-containing unsaturated. monomer(s), if present in a content
of more than 10 percent by weight, may undergo polymerization in water to
cause thickening (increase in viscosity), because such
carboxyl-Containing unsaturated monomers (e.g., acrylic acid) are
generally soluble in water. In addition, the pressure-sensitive adhesive
layer in this case may have, after its formation, increasing interaction
with a functional group on the surface of a deflector plate (an adherend)
and thereby have an increasing adhesive strength with time, and this may
impede removal of the pressure-sensitive adhesive layer from the
adherend. In contrast, the carboxyl-containing unsaturated monomer (s) if
present in a content of less than 0.5 percent by weight, may cause the
emulsion particles to have insufficient mechanical stability and may
invite insufficient adhesion (anchoring capability) between the
pressure-sensitive adhesive layer and the substrate to cause adhesive
residue.

[0036] Monomer components constitutive monomers) constituting the acrylic
emulsion polymer may further include one or more additional monomer
components for imparting as specific function, in combination with the
(meth) acrylic alkyl ester(s) and the carboxyl-containing unsaturated
monomer (s) As such monomer components, amido-containing monomers (e.g.,
(meth) acrylamide, N,N-diethyl(meth)acrylamide, and
N-isopropyl(meth)acrylamide) and/or amino-containing monomers (e.g.,
N,N-dimethylaminoethyl (meth) acrylate and N,N-dimethylaminopropyl (meth)
acrylate) may he added (used) in an amount per each category of from 0.1
to 15 percent by weight, for the purpose of increasing cohesive strength.
Typically for adjusting refractive index and/or for improving
reworkability, (meth) acrylic aryl esters such as phenyl (meth) acrylate;
vinyl esters such as vinyl acetate and vinyl propionate; and styrenic
monomers such as styrene may be added (used) in an amount per each
category of 15 percent by weight or less. For improving crosslinking in
the emulsion particles and improving cohesive strength, epoxy-containing
monomers such as glycidyl (meth) acrylate and allyl glycidyl ether;
and/or multifunctional monomers such as trimethylolpropane
tri(meth)acrylate and divinylbenzene may be added (used) in an amount per
each category of less than 5 percent by weight. For forming hydrazide
crosslinks with a combination use of a hydrazide crosslinking agent and
thereby particularly improving less-staining properties, keto-containing
unsaturated monomers such as diacetoneacrylamide (DAM), allyl
acetoacetate, and 2-(acetoacetoxy)ethyl (meth) acrylate may be added
(used) in an amount of less than 10 percent by weight (preferably from
0.5 to 5 percent by weight).

[0037] In addition, a hydroxyl-containing unsaturated monomer may be used
as the additional monomer component. Examples thereof include
2-hydroxyethyl (meth) acrylate 2-hydroxypropyl (meth) acrylate,
4-hydroxybutyl (meth) acrylate, 6-hydroxyhexyl (meth) acrylate,
8-hydroxyoctyl (meth) acrylate, 10-hydroxydecyl (meth) acrylate,
12-hydroxylauryl (meth) acrylate, (4-hydroxymethylcyclohexyl)methyl
acrylate, N-methylol(meth)acrylamide, vinyl alcohol, allyl alcohol,
2-hydroxyethyl vinyl ether, 4-hydroxybutyl vinyl ether, and diethylene
glycol monovinyl ether. The amount of such hydroxyl-containing
unsaturated monomers to he added (to be used) is preferably minimized
from the viewpoint of further reducing whitening staining. Specifically,
the constitutive monomers for constituting the acrylic emulsion polymer
(A) contain a hydroxyl-containing unsaturated monomer in an amount of
preferably less than 1 percent by weight, more preferably less than 0.1
percent by weight, and furthermore preferably, the constitutive monomers
contain substantially no hydroxyl-containing unsaturated monomer (e.g.,
in a content of less than 0.05 percent by weight). However, such a
hydroxyl-containing unsaturated monomer may be added (used) in an amount
of from about 0.01 to about 10 percent by weight when used for the
purpose of introducing crosslinking points typically of crosslinking
between hydroxyl group and isocyanate group or crosslinking of a metal
crosslinking agent.

[0038] The amounts of the additional monomer components to be added (to be
used) are indicated as contents based on total amount of constitutive
monomers (total constitutive monomers) constituting the acrylic emulsion
polymer (A) (as 100 percent by weight).

[0039] In particular, at least one monomer selected from the group
consisting of methyl methacrylate, isobornyl acrylate,
N,N-diethylacrylamide, and vinyl acetate is preferably used as a monomer
component (constitutive monomer) constituting the acrylic, emulsion
polymer (A) from the viewpoint of improving the appearance of a
pressure-sensitive adhesive sheet (pressure-sensitive adhesive layer)
obtained from the pressure-sensitive adhesive composition of the present
invention. Among them, methyl methacrylate is particularly preferably
used. Though not critical, the content of the monomer(s) (the monomer(s)
selected from the group consisting of methyl methacrylate, isobornyl
acrylate, N,N-diethylacrylamide, and vinyl acetate) is preferably from
0.5 to 15 percent by weight, more preferably from 1 to 10 percent by
weight, and furthermore preferably from 2 to 5 percent by weight, based
on total amount of constitutive monomers (total constitutive monomers)
constituting the acrylic emulsion polymer (A) (as 100 percent by weight).
The monomer(s), if present in a content of less than 0.5 percent by
weight, may not effectively help the pressure-sensitive adhesive layer to
have an improved appearance. In contrast, the monomer, if present in a
content of more than 15 percent by weight, may cause the polymer
constituting the pressure-sensitive adhesive layer to be excessively
rigid and may thereby cause insufficient adhesion. When two or more
monomers selected from the group consisting of methyl methacrylate,
isobornyl acrylate, N,N-diethylacrylamide, and vinyl acetate are
contained in the constitutive monomers constituting the acrylic emulsion
polymer (A), the total amount (total content) of methyl methacrylate,
isobornyl acrylate, N,N-diethylacrylamide, and vinyl acetate may fall
within the above-specified range.

[0040] The acrylic emulsion polymer (A) for use in the present invention
may be obtained by emulsion polymerization of the constitutive monomers
(monomer mixture) using an emulsifier and a polymerization initiator. in
addition, a chain-transfer agent may be used so as to regulate the
molecular weight of the acrylic emulsion polymer (A).

[0041] The emulsifier for use in emulsion polymerization to form the
acrylic emulsion polymer (A) is preferably a reactive emulsifier
including a radically polymerizable functional group introduced into the
molecule (reactive emulsifier containing a radically polymerizable
functional. group). Specifically, the acrylic emulsion polymer (A) is
preferably a polymer obtained by polymerization using a reactive
emulsifier containing a radically polymerizable functional group in the
molecule. Each of different emulsifiers may be used alone or in
combination.

[0042] The reactive emulsifier containing a radically polymerizable
functional group (hereinafter simply referred to as "reactive
emulsifier") is an emulsifier containing at least one radically
polymerizable functional group in the molecule (per molecule). Examples
of the reactive emulsifier include, but are not limited to, reactive
emulsifiers having radically polymerizable functional group (a) such as
vinyl group, propenyl group, isopropenyl group, vinyl ether group
(vinyloxy group), and allyl ether group (allyloxy group) Each of
different reactive emulsifiers may be used alone or in combination. Such
a reactive emulsifier, when used, is taken into the polymer, and this
reduces staining derived from the emulsifier, thus being preferred.

[0043] Examples of the reactive emulsifiers include reactive emulsifiers
each having a structure (or corresponding to the structure) of a
nonionic-anionic emulsifier (anionic emulsifier having a nonionic
hydrophilic group), except with an introduced radically polymerizable
functional group (radically reactive group) such as propenyl group or
allyl ether group. Exemplary nonionic-anionic emulsifiers include sodium
polyoxyethylene alkyl ether sulfates, ammonium polyoxyethylene alkyl
phenyl ether sulfates, sodium polyoxyethylene alkyl phenyl ether
sulfates, and sodium polyoxyethylene alkyl sulfosuccinates. Hereinafter a
reactive emulsifier having a structure corresponding to an anionic
emulsifier, except with a radically polymerizable functional group being
introduced, is referred to as an "anionic reactive emulsifier"; whereas a
reactive emulsifier having a structure corresponding to a
nonionic-anionic emulsifier, except with a radically polymerizable
functional group being introduced, is referred to as a "nonionic-anionic
reactive emulsifier."

[0044] Among them, anionic reactive emulsifiers are preferred, of which
nonionic-anionic reactive emulsifiers are more preferred, because these
emulsifiers will be taken into the polymer to improve less-staining
properties. When the water-insoluble crosslinking agent (B) is a
multifunctional epoxy crosslinking agent having an epoxy group, such an
anionic reactive emulsifier, as having catalytic activity, helps the
crosslinking agent to have higher reactivity. When no anionic reactive
emulsifier is used, the crosslinking reaction may not be completed even
through aging, and this may cause the pressure-sensitive adhesive layer
to have an adhesive strength varying with time; and, in addition,
unreacted carboxyl groups may cause the pressure-sensitive adhesive layer
to have an increasing adhesive strength to the adherend with time. The
anionic reactive emulsifiers are also preferred because they are taken
into the polymer, thereby do not precipitate to the surface of the
adherend, and cannot cause whitening staining, in contrast to quaternary
ammonium compounds (e.g., see Japanese Unexamined Patent Application
Publication (JP-A) No. 2007-31585) which are generally used as catalysts
for epoxy crosslinking agents and precipitate to the surface of the
adherend.

[0045] As the reactive emulsifiers, commercial products may also be used,
and examples thereof include products under the trade name "ADEKA REASOAP
SE-10N" (supplied by ADFKA CORPORATION), the trade name "ADEKA REASOAP
SE-20N" (supplied by ADEKA CORPORATION), the trade name "ADEKA REASOAP
SR-10" (supplied by ADEEA CORPORATION), the trade name "ADEKA REASOAP
SR:-20" (supplied by ADENA CORPORATION), the trade name "AODALON BS-10"
(supplied by Dai-ichi Kogyo Seiyaku Co., Ltd,), the trade name "AQUALON
HS-05" (supplied by Dai-ichi Kogyo Seiyaku Co., Ltd.), the trade name
"AQUALON HS-1025" (supplied by Dai-ichi Kogyo Seiyaku Co., Ltd.), and the
trade name "LATEMUL PD-104" (supplied by Kao Corporation).

[0046] The emulsifier for use herein is preferably one having a
SO42- ion concentration of 100 μg/g or less, from which
impurity ions have been removed, because such impurity ions may become a
problem. In the case of an anionic emulsifier, an ammonium salt
emulsifier is desirably used. Impurities may be removed from the
emulsifier according to a suitable process such as a process using an
ion-exchange resin, a membrane separation process, or a process of
precipitating impurities using an alcohol and removing the precipitates
by filtration.

[0047] The reactive emulsifier may be blended (used) in an amount of
preferably from 0.1 to 10 parts by weight, more preferably from 0.5 to 6
parts by weight, furthermore preferably from 1 to 4.5 parts by weight,
and most: preferably from 1 to 3 parts by weight, per 100 parts by weight
of the total amount of constitutive monomers (total constitutive
monomers) constituting the acrylic emulsion polymer (A). The reactive
emulsifier, if used in an amount of more than 10 parts by weight, may
cause the pressure-sensitive adhesive (pressure-sensitive adhesive layer)
to have an insufficient cohesive strength, resulting in increased,
staining to the adherend or resulting in contamination of the emulsifier
into the adherend. In contrast, the reactive emulsifier, if used in an
amount of less than 0.1 part by weight, may not help to maintain stable
emulsification.

[0048] The polymerization initiator for use in emulsion polymerization to
form the acrylic emulsion polymer (A) is not limited, and examples of
such polymerization initiators usable herein include azo polymerization
initiators such as 2,2'-azobisisobutyronitrile,
2,2'-azobis(2-amidinopropane) dihydrochloride,
2,2'-azobis[2-(5-methyl-2-imidazolin-2-yl)propane] dihydrochloride,
2,2'-azobis(2-methylpropionamidine) disulfate, and
2,2'-azobis(N,N'-dimethyleneisobutylamidine); persulfate polymerization
initiators such as potassium persulfate and ammonium persulfate; peroxide
polymerization initiators such as benzoyl peroxide, t-butyl
hydroperoxide, and hydrogen peroxide; redox initiators each including a
peroxide and a reducing agent, such as redox polymerization initiators
using a peroxide and ascorbic acid in combination (e.g., a hydrogen
peroxide solution and ascorbic acid in combination), those using a
peroxide and an iron (II) salt in combination (e.g., a hydrogen peroxide
solution and an iron (II) salt in combination), and those using a
persulfate and sodium hydrogen sulfite in combination.

[0049] The amount of the polymerization initiator to be incorporated (to
be used) may be determined as appropriate according typically to the
types of the initiator and the constitutive monomers and is not limited,
but is preferably from 0.01 to 1 part by weight, and more preferably from
0.02 to 0.5 part by weight, per 100 parts by weight of the total amount
of constitutive monomers (total constitutive monomers) constituting the
acrylic emulsion polymer (A).

[0050] Polymerization to form the acrylic emulsion polymer (A) may employ
a chain-transfer agent so as to regulate the molecular weight of the
acrylic emulsion polymer (A). The chain-transfer agent is not limited,
can be any of known or customary chain-transfer agents, and examples
thereof, include lauryl mercaptan, glycidyl mercaptan, mercaptoacetic
acid, 2-mercaptoethanol, thioglycolic acid, 2-ethylhexyl thioglycolate,
and 2,3-dimercapto-1-propanol. Each of different chain-transfer agents
may be used alone or in combination. The chain-transfer agent(s) may be
incorporated (used) in an amount of preferably from 0.001 to 0.5 part by
weight, per 100 parts by weight of the total amount of constitutive
monomers (total constitutive monomers) constituting the acrylic emulsion
polymer (A).

[0051] Emulsion polymerization of the acrylic emulsion polymer (A) may be
performed, by any process not limited, such as regular batch
polymerization, continuous dropping polymerization, or portion-wise
dropping polymerization. From the viewpoint of less staining (improving
less-staining properties), the emulsion polymerization is preferably
performed according to batch polymerization at a low, temperature (e.g.,
55° C. or lower, and more preferably 30° C. or lower). This
is probably because, polymerization, when performed under these
conditions, is liable to give a polymer including high-molecular-weight
components in large amounts with less amounts of low-molecular-weight
components, and this may reduce staining.

[0052] The acrylic emulsion polymer (A) is a polymer including, as
essential constitutional units, constitutional units derived from the
(meth)acrylic alkyl ester, and constitutional units derived from the
carboxyl-containing unsaturated monomer. The constitutional units derived
from the (meth) acrylic alkyl ester are contained in the acrylic emulsion
polymer (A) in a content of preferably from 70 to 99.5 percent by weight,
more preferably from 85 to 99 percent by weight, and furthermore
preferably from 91. to 98 percent by weight. The constitutional units
derived from the carboxyl-containing unsaturated monomer are contained in
the acrylic emulsion polymer (A) in a content of preferably from 0.5 to
10 percent by weight, more preferably from 1 to 5 percent by weight, and
furthermore preferably from 2 to 4 percent by weight.

[0053] The acrylic emulsion polymer (A) has a solvent-insoluble content
(content of solvent-insoluble matter, also referred to as "gel fraction")
of preferably 70% (percent by weight) or more, more preferably 75 percent
by weight or more, and furthermore preferably 80 percent by weight or
more The acrylic emulsion polymer (A), if having a solvent-insoluble
content of less than 70 percent by weight, may contain large amounts of
low-molecular-weight components, such large amounts of
low-molecular-weight components may not be sufficiently reduced and may
thereby remain in the pressure-sensitive adhesive layer by the action of
crosslinking alone. This may cause adherend staining derived typical from
low-molecular-weight components or may cause the pressure-sensitive
adhesive layer to have an excessively high adhesive strength. The
solvent-insoluble content may be controlled by choosing or setting the
polymerization initiator, reaction temperature, and types of the
emulsifier and constitutive monomers. Though not limited, the upper limit
of the solvent-insoluble content is, for example, about 99 percent by
weight.

[0054] As used herein the "solvent-insoluble content" of the acrylic
emulsion polymer (A) refers to a value as determined by calculation
according to the following "method for measuring solvent-insoluble
content."

Method for Measuring Solvent-Insoluble Content

[0055] About 0.1 g of the acrylic emulsion polymer (A) is sampled to give
a specimen, the specimen is covered, by a porous tetrafluoroethylene
sheet (supplied by Nitta Denko Corporation under the trade name
"NTF1122") having an average pore size of 0.2 μm, tied with a kite
string, the weight of the resulting article is measured and defined as a
"weight before immersion." The weight before immersion is the total
weight of the acrylic emulsion polymer (A) (the sampled specimen), the
tetrafluoroethylene sheet, and the kite string. Independently, the total
weight of the tetrafluoroethylene sheet and the kite string is measured
and defined as a tare weight.

[0056] Next, the specimen acrylic emulsion polymer (A) covered by the
tetrafluoroethylene sheet and tied with the kite string (this article is
hereinafter referred to as "sample") is placed in a 50-ml vessel filled
with ethyl acetate, and left stand therein at 23° C. for 7 days.
The sample (after treatment with ethyl acetate) is retrieved from the
vessel, transferred to an aluminum cup, dried in a drier at 130°
C. for 2 hours to remove ethyl acetate, and the weight of the resulting
sample is measured and defined as a weight after immersion.

[0057] The solvent-insoluble content is calculated according to the
following equation:

Solvent-insoluble content (percent by weight)=(a-b)/(c-b)×100 (1)

wherein "a" represents the weight after immersion; "b" represents the
tare weight; and "c" represents the weight before immersion.

[Water-Insoluble Crosslinking Agent (B)]

[0058] The water-insoluble crosslinking agent (B) for use in the
pressure-sensitive adhesive composition of the present invention is a
water-insoluble compound and is a compound having two or more (e.g., two
to six) functional groups reactive with carboxyl group (hereinafter also
referred to as carboxyl-reactive functional groups) in the molecule (per
molecule). Though not critical, the number of such carboxyl-reactive
functional groups per molecule is preferably from 3 to 5. With an
increasing number of carboxyl-reactive functional groups per molecule,
the pressure-sensitive adhesive composition undergoes denser
crosslinking, specifically the polymer constituting the
pressure-sensitive adhesive layer has a denser crosslinking structure.
This enables the prevention of wet spread of the pressure-sensitive
adhesive layer after its formation. In addition, such dense crosslinking
structure, constrains the polymer constituting the pressure-sensitive
adhesive layer and thereby prevents increase in adhesive strength between
the pressure-sensitive adhesive layer and the adherend with time due to
segregation of the functional groups (carboxyl groups) in the
pressure-sensitive adhesive layer to the surface in contact with the
adherend. In contrast, the water-insoluble crosslinking agent (B), if
having a number of carboxyl-reactive functional groups per molecule of
more than 6, may cause the formation of a gelled substance.

[0059] Exemplary carboxyl-reactive functional groups to be contained in
the water-insoluble crosslinking agent (B) include, but are not limited
to, epoxy groups, isocyanate groups, and carbodiimide groups. Among them,
epoxy groups are preferred from the viewpoint of reactivity. Of such
epoxy groups, glycidylamino group is more preferred, because the
glycidylamino group is highly reactive, thereby less causes a substance
unreacted in the crosslinking reaction to remain, thus contributes to
less-staining properties, and prevents increase with time in adhesive
strength to the adherend due to the presence of unreacted carboxyl groups
in the pressure-sensitive adhesive layer. Specifically, the
water-insoluble crosslinking agent (B) is preferably any of epoxy
crosslinking agents each having an epoxy group, of which crosslinking
agents each having a glycidylamino group (glycidylamino crosslinking
agents) are more preferred. Though not critical, the water-insoluble
crosslinking agent (B), when being an epoxy crosslinking agent
(particularly glycidylamino crosslinking agent), contains epoxy groups
(particularly glycidylamino groups) in a number of preferably 2 or more
(e.g., from 2 to 6), and more preferably from 3 to 5 per molecule.

[0060] The water-insoluble crosslinking agent (B) is a water-insoluble
compound. As used herein the term "water-insoluble" refers to that the
compound (crosslinking agent) has a solubility at 25° C. in 100
parts by weight of water (weight of the compound (crosslinking agent)
soluble in 100 parts by weight of water) of 5 parts by weight or less.
The solubility is preferably 3 parts by weight or less, and more
preferably 2 parts by weight or less. Such a water-insoluble crosslinking
agent, when used, hardly causes whitening staining and thereby helps the
pressure-sensitive adhesive layer to have improved less-staining
properties. The whitening staining is caused. on the adherend in a
high-humidity environment by a crosslinking agent which has not undergone
crosslinking and remains in the resulting polymer. Crosslinking, when
performed by using a water-soluble crosslinking agent alone as a
crosslinking agent, tends to cause whitening staining in a high-humidity
environment, because a residual crosslinking agent is liable to be
dissolved in water and to migrate into the adherend. In addition, a
water-insoluble crosslinking agent more contributes to the crosslinking
reaction (reaction with carboxyl group) and more effectively suppresses
increase in adhesive strength with time, as compared to a water-soluble
crosslinking agent. A water-insoluble crosslinking agent has high
reactivity in a crosslinking reaction, thereby allows the crosslinking
reaction to proceed. rapidly during aging, and suppresses increase in
adhesive strength to the adherend with time Such increase in adhesive
strength is caused by unreacted carboxyl. groups in the
pressure-sensitive adhesive layer.

[0061] The solubility of the crosslinking agent in water may for example
be measured in the following manner.

Method for Measuring Solubility in Water

[0062] Water (25° C.) and the crosslinking agent in an equal weight
are mixed with a stirrer at 300 revolutions per minute (rpm) for 10
minutes, and the mixture is separated into an aqueous phase and an oily
phase by centrifugal separation. Next, the aqueous phase is collected,
dried at 120° C. for 1 hour, a weight loss on drying is
determined, from which a non-volatile content (weight of non-volatile
components in parts by weight per 100 parts by weight of water) in the
aqueous phase is determined.

[0063] Specific examples of the water-insoluble crosslinking agent (B)
include glycidylamino crosslinking agents such as
1,3bis(N,N-diglycidylaminomethyl)cyclohexane (e.g., a product supplied by
Mitsubishi Gas Chemical Company, Inc. under the trade name "TETRAD-C")
[having a solubility of 2 parts by weight or less per 100 parts by weight
of water at 25° C.], 1,3-bis(N,N-diglycidylaminomethyl)benzene
(e.g., a product supplied by Mitsubishi Gas Chemical Company, Inc. under
the trade name "TETRAD-X") [having a solubility of 2 parts by weight or
less per 100 parts by weight of water at. 25° C.]; and other epoxy
crosslinking agents such as Tris (2,3-epoxypropyl)isecyanurate (e.g., a
product supplied by Nissan Chemical Industries, Ltd. under the trade name
"TEPIC-G") [having a solubility of 2 parts by weight or less per 100
parts by weight of water at 25° C.]

[0064] Incorporation of the water-insoluble crosslinking agent (B) during
the preparation of the pressure-sensitive adhesive composition. of the
present invention may be performed by adding (incorporating) the
water-insoluble crosslinking agent (B) as intact, or by adding the
water-insoluble cross inking agent (B) after being dissolved in and/or
diluted with an organic solvent. In the latter case, however, the amount
of the organic solvent is preferably minimized. Addition of the
water-insoluble crosslinking agent (B) after being emulsified with an
emulsifier is not preferred, because the emulsifier tends to bleed out
and to cause staining (particularly whitening staining).

[0065] The amount (content in the pressure-sensitive adhesive composition
of the present invention) of the water-insoluble crosslinking agent(s)
(M) is preferably such that the number of moles of carboxyl-reactive
functional groups of the water-insoluble crosslinking agent(s) (B) be
from 0.3 to 1.3 moles per 1 mole of carboxyl groups of the
carboxyl-containing unsaturated monomer (s) used as a constitutive
monomer for the acrylic emulsion polymer (A). Specifically, the ratio
[(carboxyl-reactive functional groups)/(carboxyl groups)] (molar ratio)
of the "total number of moles of carboxyl-reactive functional groups in
total water-insoluble crosslinking agents (B)" to the "total number of
moles of carboxyl groups in total carboxyl-containing unsaturated
monomers used as constitutive monomers for the acrylic emulsion polymer
(A)" is preferably from 0.3 to 1.3, more preferably from 0.4 to 1.1, and
furthermore preferably from 0.5 to 1.0. When the ratio
[(carboxyl-reactive functional groups)/(carboxyl groups)] is less than
0.3, unreacted carboxyl groups may remain in a large amount in the
pressure-sensitive adhesive layer, and this may cause increase in
adhesive strength with time due to interactions between the carboxyl
groups and the adherend. In contrast, if the ratio is more than 1.3,
unreacted water-insoluble crosslinking agent (s) (B) may remain in a
large amount in the pressure-sensitive adhesive layer, and this may cause
appearance defects.

[0066] Particularly when the water-insoluble crosslinking agent (B) is an
epoxy crosslinking agent, the ratio [(epoxy groups)/(carboxyl groups)]
(molar ratio) is preferably from 0.3 to 1.3, more preferably from 0.4 to
1.1, and furthermore preferably from 0.5 to 1.0, where the carboxyl
groups are carboxyl groups of the carboxyl-containing unsaturated monomer
(s) used as a constitutive monomer for the acrylic emulsion polymer (A).
When the water-insoluble crosslinking agent (B) is a glycidylamino
crosslinking agent, the ratio [(glycidylamino groups)/(carboxyl groups)]
(molar ratio) preferably falls within the above range, where the carboxyl
groups are carboxyl groups of the carboxyl-containing unsaturated
monomer(s) used as a constitutive monomer for the acrylic emulsion
polymer (A).

[0067] For example, when a water-insoluble crosslinking agent (B) having a
functional-group equivalent, of carboxyl-reactive functional groups of
110 (g/eq) is added (incorporated) in an amount of 4 grams to the
pressure-sensitive adhesive composition, the number of moles of
carboxyl-reactive functional groups possessed by the water-insoluble
crosslinking agent (B) may for example be calculated according to the
following equation:

Number of moles of carboxyl-reactive functional groups of
water-insoluble crosslinking agent (B)=[Amount of water-insoluble
crosslinking agent (B) to be added. (incorporated)]/[Functional-group
equivalent]=4/110

[0068] Further for example, when an epoxy crosslinking agent having an
epoxy equivalent of 110 (g/eq) is added (incorporated) in an amount of 4
grams as the water-insoluble crosslinking agent (B), the number of moles
of epoxy groups of the epoxy crosslinking agent may for example be
calculated according to the following equation:

[0069] Number of moles of epoxy groups of epoxy crosslinking agent [Amount
of epoxy crosslinking agent to be added (incorporated)]/[Epoxy
equivalent]=4/110

[Carboxylic-Acid-Containing Acrylic Polymer (C)]

[0070] The carboxylic-acid-containing acrylic polymer (C) in the
pressure-sensitive adhesive composition of the present invention is a
polymer formed from a (meth) acrylic acid (either acrylic acid or
methacrylic acid, or both) as an essential constitutive monomer.

[0071] The total amount of acrylic acid and methacrylic acid (total sum of
the acrylic acid content and the methacrylic acid content (total
content)). is typically 11 percent by weight or more (e.g., from 11 to
100 percent by weight), preferably 21 percent by weight or more (e.g.,
from 21 to 95 percent by weight), more preferably 31 percent by weight.
or more (e.g., from 31 to 90 percent by weight), and furthermore
preferably 41 percent by weight or more, and most preferably 45 percent
by weight or more, based on the total amount. (100 percent by weight) of
constitutive monomers constituting the carboxylic-acid-containing acrylic
polymer (C). If the total content of acrylic acid and methacrylic acid is
less than 11 percent by weight, sufficient thickening effects may not be
obtained, and this may not provide an effect of suppressing dimple
defects derived from the water-insoluble crosslinking agent.

[0074] The carboxylic-acid-containing acrylic polymer (C) may be prepared
by polymerizing the above-mentioned monomer component (s) according to a
known or customary polymerization process. Exemplary polymerization
processes for the carboxylic-acid-containing acrylic polymer (C) include
solution polymerization, emulsion polymerization, batch polymerization,
and polymerization through irradiation with an active energy ray
(active-energy-ray polymerization).

[0075] The carboxylic-acid-containing acrylic polymer (C) is a polymer
including constitutional units derived from (meth) acrylic acid as
essential constitutional units. The carboxylic-acid-containing acrylic
polymer (C) may have a total amount of constitutional units derived from
acrylic acid and constitutional units derived from methacrylic acid
(total sum of the content of constitutional units derived from acrylic
acid and the content of constitutional units derived from methacrylic
acid (total content)) of 11 percent by weight or more (e.g., from 11 to
100 percent by weight), preferably 21 percent by weight or more (e.g.,
from 21 to 95 percent by weight), more preferably 31 percent by weight or
more (e.g., from 31 to 90 percent by weight), and furthermore preferably
41 percent by weight or. more, and most preferably 45 percent by weight
or more. The carboxylic-acid-containing acrylic polymer (C) preferably
further includes constitutional units derived from a (meth) acrylic
ester. The total amount of constitutional units derived from acrylic acid
and constitutional units derived from methacrylic acid may be measured
typically by nuclear magnetic resonance spectrometry (NMR), gas
chromatography-mass spectrometry (GC/MS), or Fourier transform infrared
spectroscopy (FT-IR).

[0076] Though not critical, the carboxylic-acid-containing acrylic polymer
(C) has a number-average molecular weight of preferably from 10,000 to
4,000,000, and more preferably from 10,000 to 2,000,000, from the
viewpoint of less-staining properties.

[0077] Commercial products may be used as the carboxylic-acid-containing
acrylic polymer (C). Specifically, exemplary commercial products usable
herein include thickeners each containing a carboxylic-acid-containing
acrylic polymer (C), such as "Aron A-7070", "Aron A-7075", "Aron B-300K",
and "Aron B-500" each supplied by Toagosei Co., Ltd.; "SN-Thickener 630",
"SN-Thickener 634", and "SN-Thickener 640" each supplied by SAN NOPCO
LIMITED; and "PRIMAL ASE-60" supplied by Rohm and Haas Company.

[0078] The carboxylic-acid-containing acrylic polymer (C), as being
incorporated into the pressure-sensitive adhesive composition of the
present invention, helps the pressure-sensitive adhesive composition to
have a higher viscosity and to be resistant to formation of dimple
defects caused by the water-insoluble crosslinking agent, thus
suppressing defects formation.

[0079] The carboxylic-acid-containing acrylic polymer (C) is contained in
an amount (content in the pressure-sensitive adhesive composition of the
present invention) of preferably from MI to 10 parts by weight, more
preferably from 0.1 to 7 parts by weight, and furthermore preferably from
0.5 to 5 parts by weight, per 100 parts by weight of the acrylic emulsion
polymer (A). The carboxylic-acid-containing acrylic polymer (C), if in a
content of less than 0.01 part by weight, may not sufficiently prevent
appearance defects (poor appearance) due to dimple defects caused by the
water-insoluble crosslinking agent. In contrast, the
carboxylic-acid-containing acrylic polymer (C), if in a content of more
than 10 parts by weight, may cause the pressure-sensitive adhesive
composition to have an excessively high viscosity, and this may tend to
cause troubles upon coating of the pressure-sensitive adhesive
(composition).

[0080] The removable water-dispersible acrylic pressure-sensitive adhesive
composition according to the present invention contains an acrylic
emulsion polymer (A), a water-insoluble crosslinking agent (B), and a
carboxylic-acid-containing acrylic polymer (C) as essential components,
as is described above. The composition may further contain another
crosslinking agent than the water-insoluble crosslinking agent (B)
(hereinafter also referred to as "additional crosslinking agent(s)"), and
other additives according to necessity.

[0081] The pressure-sensitive adhesive composition of the present
invention is a water-dispersible pressure-sensitive adhesive composition.
As used herein the term "water-dispersible" refers to that the substance
in question is capable of dispersing in an aqueous medium. Specifically,
the pressure sensitive adhesive composition of the present invention is a
pressure-sensitive adhesive composition capable of dispersing in an
aqueous medium. The aqueous medium is a medium (dispersion medium)
including water as an essential component and may be composed of water
alone or a mixture of water and a water-soluble(miscible) organic
solvent. The pressure-sensitive adhesive composition of the present
invention may also be in the form of a fluid dispersion using, for
example, the aqueous medium.

[0083] The hydrazide crosslinking agent may be a commercial product. such
as a product supplied by Tokyo Chemical industry Co., Ltd. under the
trade name "Adipic Acid Dihydrazide (Reagent)" and a product supplied by
Wako Pure Chemical Industries, Ltd, under the trade name "Adipoyl
Dihydrazide (Reagent),"

[0084] The amount (content in the pressure-sensitive adhesive composition
of the present invention) of the hydrazide crosslinking agent is
preferably from 0.025 to 2.5 moles, more preferably from 0.1 to 2 moles,
and furthermore preferably from 0.2 to 1.5 moles, per 1 mole of keto
groups of the keto-containing unsaturated monomer used as a constitutive
monomer for the acrylic emulsion polymer (A). The hydrazide crosslinking
agent, if in an amount of less than 0.025 mole, may not exhibit
sufficient effects of its addition, and this may cause the
pressure-sensitive adhesive layer or pressure-sensitive, adhesive sheet
to be peeled off heavily and may tend to cause whitening staining of the
adherend due to low-molecular-weight components remained in the polymer
constituting the pressure-sensitive adhesive layer. In contrast, the
hydrazide crosslinking agent, if in an amount of more than 2.5 moles, may
cause staining due to an unreacted crosslinking agent component.

[0085] From the viewpoint of less-staining properties, the
pressure-sensitive adhesive composition of the present invention is
preferably not incorporated with quaternary ammonium salts, and more
preferably not incorporated with quaternary ammonium compounds,
Accordingly, the pressure-sensitive adhesive composition of the present
invention preferably contains substantially no quaternary ammonium salt,
and more preferably contains substantially no Quaternary ammonium
compound. These compounds are generally used typically as catalysts for
improving reactivity of epoxy crosslinking agents. These compounds,
however, are not integrated into the polymer constituting the
pressure-sensitive adhesive layer, are capable of migrating freely in the
pressure-sensitive adhesive layer, and are tend to precipitate to the
adherend surface. For these reasons, these compounds, if contained in the
pressure-sensitive adhesive composition, may often cause whitening
staining and may thereby impede development of less-staining properties.
Specifically, the pressure-sensitive adhesive composition of the present
invention has a quaternary ammonium salt content of preferably less than
0.1 percent by weight, more preferably less than 0.01 percent by weight,
and furthermore preferably less than 0.005 percent by weight, based on
the total amount (non-volatile content) of the pressure-sensitive
adhesive composition. In addition, the pressure-sensitive adhesive
composition preferably has a quaternary ammonium compound content falling
within the above-specified range.

[0086] Specifically, exemplary quaternary ammonium salts include, but are
not limited to, compounds each represented by the following formula:

##STR00001##

[0087] In the formula, R1, R2, R3, and R4 each
independently represent not hydrogen atom but an alkyl group, an aryl
group, or a group derived from them (e.g., a substituted alkyl group or
aryl group); and X.sup.- represents a counter ion.

[0089] Also from the viewpoint of less-staining properties, the
pressure-sensitive adhesive composition of the present invention is
preferably not incorporated with tertiary amines and imidazole compounds,
as with the quaternary ammonium salts (or quaternary ammonium compounds).
Such tertiary amines and imidazole compounds are generally used typically
as catalyst for improving reactivity of epoxy crosslinking agents.
Accordingly, the pressure-sensitive adhesive composition of the present
invention preferably contains substantially no tertiary amine and
substantially no imidazole compound. Specifically, the pressure-sensitive
adhesive composition of the present invention, has a content of tertiary
amines and imidazole compounds (total content of tertiary amines and
imidazole compounds) of preferably less than 0.1 percent by weight, more
preferably less than 0.01 percent by weight, and furthermore preferably
less than 0.005 percent by weight, based on the total amount
(non-volatile content) of the pressure-sensitive adhesive composition.

[0091] In addition, the pressure-sensitive adhesive composition of the
present invention preferably contains substantially no hydrophobic
silica. Specifically, the pressure-sensitive adhesive composition of the
present invention has a hydrophobic silica content of preferably less
than 5×10-4 percent by weight, more preferably less than
1×10-4 percent by weight, furthermore preferably less than
1×10-5 percent by weight, and most preferably 0 percent by
weight, based on the total amount (non-volatile content) of the
pressure-sensitive adhesive composition. Hydrophobic silica, if contained
in the pressure-sensitive adhesive composition, may form secondary
aggregates, and the resulting silica particles (aggregates) may cause a
defect (appearance defect). In addition, upon filtration of the
pressure-sensitive adhesive composition typically through a filter, the
filter may be clogged with the silica particles, resulting in
insufficient production efficiency.

[0092] The pressure-sensitive adhesive composition of the present
invention may further contain various additives within a range not
adversely affecting less-staining properties, in addition to the above
components. Exemplary additives include pigments, fillers, dispersing
agents, plasticizers, stabilizers, antioxidants, ultraviolet absorbents,
ultraviolet stabilizers, age inhibitors, and antiseptic agents.

[0093] The pressure-sensitive adhesive composition of the present
invention may be prepared by mixing the acrylic emulsion polymer (A), the
water-insoluble crosslinking agent (B), and the
carboxylic-acid-containing acrylic polymer (C). Where necessary,
additional crosslinking agents and other additives may be mixed. The
mixing may be performed according to any known or customary technique for
mixing an emulsion, but is preferably performed, for example, by stirring
with a stirrer. Though stirring conditions are not limited, the stirring
is performed, for example, at a temperature (stirring temperature) of
preferably from 10° C. to 50° C., and more preferably from
20° C. to 35° C. for a duration (stirring time) of
preferably from 5 to 30 minutes, and more preferably from 10 to 20
minutes at a number of revolutions in stirring of preferably from 10 to
2000 rpm, and more preferably from 30 to 1000 rpm.

[0094] The pressure-sensitive adhesive composition thus obtained is
applied to at least one side of a substrate. (also referred to as a "base
material" or "supporting substrate"), dried according to necessity to
form a pressure-sensitive adhesive layer, and thereby yields a
pressure-sensitive adhesive sheet according to the present invention
which is a pressure-sensitive adhesive sheet including a substrate and,
on at least one side thereof, a pressure-sensitive adhesive layer ferried
from the pressure-sensitive adhesive composition of the present
invention. Crosslinking is performed typically by heating the
pressure-sensitive adhesive sheet after dehydration and drying in the
drying step. The pressure-sensitive adhesive layer herein is preferably
provided by a so-called direct application technique in which the
pressure-sensitive adhesive composition is directly applied to the
substrate surface, as described above. The direct application technique
is preferred because the pressure-sensitive adhesive layer herein has a
high solvent-insoluble content and, if formed by a transfer process,
where a pressure-sensitive adhesive layer is once provided on a release
film and then transferred onto the substrate, may not exhibit sufficient
anchoring capability (adhesion) to the substrate.

[0095] Though not critical, the pressure-sensitive adhesive layer (after
crosslinking) in the pressure-sensitive adhesive sheet according to the
present invention has a thickness of preferably from 1 to 50 μm, more
preferably from 1 to 35 μm, and furthermore preferably from 3 to 25
μm.

[0096] The pressure-sensitive adhesive layer (after crosslinking) in the
pressure-sensitive adhesive sheet according to the present invention has
a solvent-insoluble content (gel fraction) of preferably 90% (percent by
weight) or more, and more preferably 95 percent by weight or more. The
pressure-sensitive adhesive layer, if having a solvent-insoluble content
of less than 90 percent by weight, may cause increased transfer of
contaminants to the adherend to cause whitening staining or may have
insufficient removability to cause heavy peeling off (insufficient
removability). The solvent-insoluble content of the pressure-sensitive
adhesive layer (after crosslinking) may be measured by the procedure as
with the method for measuring the solvent-insoluble content of the
acrylic emulsion polymer (A). Specifically, the solvent-insoluble content
herein may be measured by the procedure corresponding to the "method for
measuring the solvent-insoluble content" except that the term "acrylic
emulsion polymer (A)" is read as "pressure-sensitive adhesive layer
(after crosslinking)."

[0097] Though not critical, the pressure-sensitive adhesive layer (after
crosslinking) in the pressure-sensitive adhesive sheet according to the
present invention has an elongation at break (elongation at rupture) in a
tensile test of preferably 200% or less, more preferably 150% or less,
furthermore preferably 130% or less, still, more preferably from 40% to
120%, and most preferably from 60% to 115%, from the viewpoint of degree
of crosslinking of the pressure-sensitive adhesive layer. The elongation
at break is an index of the degree of crosslinking of the
pressure-sensitive adhesive layer end, if being 200% or less, the polymer
constituting the pressure-sensitive adhesive layer may have a dense
crosslinking structure. This prevents wet spread of the formed
pressure-sensitive adhesive layer and suppresses increase in adhesive
strength to the adherend with time. In addition, such dense crosslinking
structure constrains the polymer constituting the pressure-sensitive
adhesive layer and thereby prevents increase in adhesive strength to the
adherend with time due to segregation of the functional groups (carboxyl
groups) in the pressure-sensitive adhesive layer to the surface in
contact with the adherend.

[0098] Though not critical, the acrylic polymer (after crosslinking)
constituting the pressure-sensitive adhesive layer in the
pressure-sensitive adhesive sheet according to the present invention has
a glass transition temperature of preferably from -70° C. to
-10° C., more preferably from -70° C. to -20° C.,
furthermore preferably from -70° C. to -40° C., and most
preferably from -70° C. to -60° C. The acrylic polymer, if
having a glass transition temperature of higher than -10° C., may
show insufficient adhesive strength and may thereby cause gap and/or
separation typically upon working. The acrylic polymer, if having a glass
transition temperature of lower than -70° C., may cause heavy
peeling when peeled off at higher peel rates (at higher tensile speeds)
and may thereby cause insufficient working efficiency. The glass
transition temperature of the polymer (after crosslinking) may also be
regulated typically by the monomer composition upon preparation of the
acrylic emulsion polymer (A).

[0099] The substrate for use in the pressure-sensitive adhesive sheet
according to the present invention is preferably a plastic substrate
(e.g., a plastic film or plastic sheet) for allowing the
pressure-sensitive adhesive sheet to have high transparency. Exemplary
materials for plastic substrates include, but are not limited to,
transparent resins including polyolefins (polyolefin resins) such as
polypropylenes and polyethylenes; polyesters (polyester resins) such as
poly(ethylene terephthalate s (PETs); polyearbonates; polyamides;
polyamides; acrylic resins; polystyrenes; acetate resins; poly (ether
sulfene)s; and triacetylcellulose. Each of different resins may be used
alone or in combination. Though not limited, polyester resins and
polyolefin resins are preferred among these resins, of which PETs,
polypropylenes, and polyethylenes are more preferred from the viewpoints
of productivity and formability (moldability). Specifically, as the
substrate, polyester films and polyolefin films are preferred, of which
PET films, polypropylene films, and polyethylene films are more
preferred. Though not limited, examples of the polypropylenes include
homopolypropylenes as homopolymers; random polypropylenes as
α-olefin random copolymers; and block polypropylenes as
α-olefin block copolymers. Exemplary polyethylenes include
low-density polyethylenes (LDPEs), high-density polyethylenes (HDPEs),
and linear low-density polyethylenes (L-LDPEs), Each of these may be used
alone or in combination. Though not critical, the substrate has a
thickness of preferably from 10 to 150 μm, and more preferably from 30
to 100 μm.

[0100] The surface of the substrate where the pressure-sensitive adhesive
layer is to he provided has preferably undergone an adhesion-promoting
treatment such as acid. treatment, base treatment, primer treatment,
corona treatment, plasma treatment, or ultraviolet ray treatment
typically for improving the adhesion to the pressure-sensitive adhesive
layer, The pressure-sensitive adhesive sheet may further include an
intermediate layer between the substrate and the pressure-sensitive
adhesive layer. The intermediate layer has a thickness of preferably from
0.05 to 1 μm, and more preferably from 0.1 to 1 μm.

[0101] Though not critical, the pressure-sensitive adhesive sheet
according to the present invention has an adhesive strength of preferably
from 0.01 to 5 N/25 mm, more preferably from 0,02 to 3 N/25 mm,
furthermore preferably from 0.03 to 2 N/25 mm, and most preferably from
0.04 to 1 N/25 mm at a tensile speed of 0.3 m/min with respect to a
deflector plate (triacetylcellulose (TAC) plate) (one having an
arithmetic mean surface roughness Ra of 50 nm. or less). The adhesive
strength. is the peel strength measured in a 180-degree peel test upon
peeling of the pressure-sensitive adhesive sheet which has been affixed
to the deflector plate. The pressure-sensitive adhesive sheet, if having
an adhesive strength of more than 5 N/25 mm, may become resistant to
peeling off (removal) during the production processes typically of a
deflector plate or a liquid crystal display device and may thereby cause
insufficient productivity and handleability. The pressure-sensitive
adhesive sheet, if having an adhesive strength of less than 0.01 N/25 mm,
may suffer from gap or separation during the production processes and may
not satisfactorily serve, to protect the product as a surface-protecting
pressure-sensitive adhesive sheet. The arithmetic mean surface roughness
Re may be measured typically by using "P-15" (stylus-type surface
profilometer) supplied by KLA-Tencor Corporation. Though not limited, the
surface roughness (arithmetic, mean surface roughness Re) may be measured
in one scanning pass at a measurement length of 1000 μm and a scanning
speed of 50 μm/second under a load of 2 mg.

[0102] The pressure-sensitive adhesive sheet according to the present
invention satisfactorily less causes whitening staining of the adherend.
This may be evaluated, for example, in the following manner. The
pressure-sensitive adhesive sheet is affixed to a deflector plate (the
trade name "SEG1425DUHC", supplied by Nitto Denko Corporation) at 0.25
MPa and 0.3 m/min, left stand at 80° C. for 4 hours, and then
removed from the deflector plate. The deflector plate after the removal
of the pressure-sensitive adhesive sheet is further left stand in an
ambient. temperature of 23° C. and relative humidity of 90% for 12
hours, and the surface thereof is observed. In this observation, it is
preferred that no whitening is observed in the deflector plate surface.
The pressure-sensitive adhesive sheet, if causing whitening of the
adherend, deflector plate under humid conditions. (high-humidity
conditions) after affixation and removal of the pressure-sensitive
adhesive sheet, has insufficient less-staining properties to be used as a
surface-protecting film for optical elements.

[0103] The pressure-sensitive adhesive sheet according to the present
invention may be formed into a roll. In this case, the sheet may be wound
as a roll while the pressure-sensitive adhesive layer is protected with a
release film (separator). The pressure-sensitive adhesive sheet may have,
on its backside (side opposite to the pressure-sensitive adhesive layer),
a backing layer (e.g., release layer or antifouling layer) through a
release treatment and/or an antifouling treatment typically with a
release agent such as a silicone, fluorine -containing, long-chain alkyl,
or fatty acid amide release agent, or a silica powder. The
pressure-sensitive adhesive sheet according to the present invention.
particularly preferably has a structure of [(pressure-sensitive adhesive
layer)/(substrate)/(hacking layer)].

[0104] The pressure-sensitive adhesive sheet according to the present
invention more preferably has undergone an antistatic treatment. The
antistatic treatment may be performed by a regular antistatic treatment
process, is not limited, but may be performed typical by a process of
providing an antistatic layer on the backside (side opposite to the
pressure-sensitive adhesive layer) of the substrate or a process of
incorporating a kneading-type antistatic agent (electrification
preventing agent) into the substrate

[0105] The antistatic layer may be provided typically by applying an
antistatic agent alone, an antistatic resin containing the antistatic
agent and a resin component, an electroconductive resin composition
containing an electroconductive material and a resin component, or an
electroconductive polymer; or by performing vapor deposition or plating
of an electroconductive material.

[0106] Examples of the antistatic agent include cationic antistatic agents
each having a cationic functional group (e.g., primary amino group,
secondary amino group, or tertiary amino group), such as quaternary
ammonium salts and pyridinium salts; anionic antistatic agents each
having an anionic functional group, such as sulfonic acid salts, sulfuric
ester salts, phosphonic acid salts, and phosphoric ester salts; amphionic
antistatic agents such as alkylbetaines and derivatives thereof,
imidazoline and derivatives thereof, and alanine and derivatives thereof;
nonionic antistatic agonts such as amino-alcohols and derivatives
thereof, glycerol and derivatives thereof, and polyethylene glycols and
derivatives thereof; as well as ironically conductive polymers obtained
by polymerization or copolymerization of a monomer or monomers each
having an ironically conductive group, as typified by the cationic
antistatic agents, anionic antistatic agents, and amphionic antistatic
agents.

[0110] Examples of the resin component for use herein include
general-purpose resins such as polyesters, acrylic resins, vinyl
polymers, urethane resins, melamine resins, and epoxy resins. When the
antistatic agent is a polymeric antistatic agent, the antistatic resin
does not have to contain the resin component. The antistatic resin may
further contain, as a crosslinking agent, a compound such as methylolated
or alkylolated melamine, urea, glyoxal, or acrylamide compound, an epoxy
compound, or an isocyanate compounds.

[0111] The antistatic layer may be formed through coating typically by
diluting the antistatic resin, electroconductive polymer, or
electroconductive resin composition with a solvent or dispersion medium
such as an organic solvent or water to form a coating composition, and
applying the coating composition to the substrate, followed by drying.
Exemplary organic solvents include methyl ethyl ketone, acetone, ethyl
acetate, tetrahydrofuran, dioxane, cyclohexanone, n-hexane, toluene,
xylene, methanol, ethanol, n-propanol, and isopropanol. Each of these may
be used alone or in combination. The coating may be performed according
to a known coating procedure such as roll coating, gravure coating,
reverse coating, roll brush coating, spray coating, air knife coating,
impregnation and curtain coating.

[0112] Though not critical, the antistatic layer (antistatic resin layer,
electroconductive polymer layer, or electroconductive resin composition
layer) formed by coating has a thickness of preferably from 0.001 to 5
μm, and more preferably from 0.005 to 1 μm.

[0114] Though not critical, the antistatic layer (electroconductive
material layer) formed by vapor deposition or plating has a thickness of
preferably from 20 to 10000 angstroms, and more preferably from 50 to
5000 angstroms.

[0115] Any of the aforementioned antistatic agents may be suitably used as
the kneading-type antistatic agent. The kneading-type antistatic agent is
incorporated in an amount of preferably 20 percent by weight or less, and
more preferably from 0.05 to 10 percent by weight, based on the total
weight (100 percent by weight) of the substrate. The way to incorporate
is not limited, as long as the kneading-type antistatic agent can be
uniformly mixed with, for example, a resin for use in a plastic
substrate, and examples thereof generally include processes typically
using a heating roil, Banbury mixer, press kneader, or double-screw
kneader.

[0116] The pressure-sensitive adhesive composition of the present
invention is a pressure-sensitive adhesive composition capable of forming
a pressure-sensitive adhesive layer excellent in adhesiveness and
removability (easy-peelability) and is used for the formation of a
(removable) pressure-sensitive adhesive layer that is used in an
application where the pressure-sensitive adhesive layer will be removed.
Specifically, the pressure-sensitive adhesive sheet having a
pressure-sensitive adhesive layer formed from the pressure-sensitive
adhesive composition of the present invention is used in applications
where the sheet will be removed. Exemplary applications include masking
tapes such as masking tapes for protection or curing in construction,
masking tapes for automobile painting, masking tapes for electronic
components (e.g., lead frames and printed circuit boards), and masking
tapes for sand blasting; surface-protecting films such as
surface-protecting films for aluminum sash, surface-protecting films for
optical plastics, surface-protecting films for optical glass,
surface-protecting films for the protection of automobiles, and
surface-protecting films for metal plates; pressure-sensitive adhesive
tapes for production, processes of semiconductor/electronic components,
such as backgrinding tapes, pellicle-fixing tapes, dicing tapes,
leadframe-fixing tapes, cleaning tapes, dedusting tapes, carrier tapes,
and cover tapes; packaging tapes for electronic appliances and electronic
components; temporal tacking tapes upon transportation; binding tapes;
and labels.

[0117] In addition, the pressure-sensitive adhesive sheet according to the
present invention less suffers from appearance defects such as "dimples"
of the pressure sensitive adhesive layer and has a good appearance. The
pressure-sensitive adhesive sheet, when used as being affixed to an
adherend, does not cause staining, such as whitening staining, of the
adherend and has excellent less-staining properties. For these reasons,
the pressure-sensitive adhesive sheet according to the present invention
is preferably used particularly for protecting the surface of optical
elements (e.g., optical plastics, optical glass, and optical films)
typically as a surface-protecting film for an optical element, in which
satisfactory appearance characteristics and less-staining properties are
required, Exemplary optical elements include deflector plates,
retardation films, anti-reflective films, wave plates, compensation
films, and brightness enhancing films for constituting panels typically
of liquid crystal displays, organic electroluminescence (organic EL)
displays, and field emission displays. However, the pressure-sensitive
adhesive sheet can also be used for other applications not limited to the
above ones and can be used typically for surface-protection,
failure-prevention, removal typically of foreign matter, or masking upon
production of microfabricated components typically of semiconductors
(semiconductor devices), circuits, printed circuit boards, masks, and
lead frames.

EXAMPLES

[0118] The present invention will, be illustrated in further detail with
reference to several working examples below. It should he noted, however,
that these examples are never construed to limit the scope of the present
invention.

Example 1

Preparation of Acrylic Emulsion Polymer (A)

[0119] A monomer emulsion was prepared by placing in a vessel 90 parts by
weight of water, constitutive monomers and an emulsifier in amounts given
in Table 1, and mixing them with stirring using a homomixer.

[0120] Next, 50 parts by weight of water, 0.01 part by weight of a
polymerization initiator (ammonium persulfate), and the above-prepared
monomer emulsion in an amount corresponding to 10 percent by weight
thereof were placed in a reactor equipped with a condenser, a nitrogen
inlet tube, a thermometer, and a stirrer, and these were subjected to
emulsion polymerization with stirring at 75° C. for 1 hour.
Thereafter 0.05 part by weight of the polymerization initiator (ammonium
persulfate) was further added, and all the residual monomer emulsion (in
an amount corresponding to 90 percent by weight) was added thereto with
stirring over 3 hours, followed by a reaction at 75° C. for 3
hours. Next, this was cooled to 30° C., combined with a 10 percent
by weight ammonia water to he adjusted to pH 8, and thereby yielded a
water dispersion of acrylic emulsion polymer (A) having a concentration
of the acrylic emulsion polymer (A) of 42 percent by weight.

[0121] A removable water-dispersible acrylic pressure-sensitive adhesive
composition was prepared by adding 3 parts by weight of an epoxy
crosslinking agent [supplied by Mitsubishi Gas Chemical Company, Inc.
under the trade name "TETRAD-C",
1,3-bis(N,N-diglycidylaminomethyl)cyclehexane, epoxy equivalents: 110,
number of functional groups: 4] as a water-insoluble crosslinking agent.
and 20 parts by weight of an aqueous solution to 238 parts by weight (100
parts by weight of the acrylic emulsion polymer) of the above-prepared
(42 percent by weight) water dispersion of the acrylic emulsion polymer
(A), and mixing them with stirring using the stirrer at 23° C., at
300 rpm. for 10 minutes. The aqueous solution had been prepared by
diluting a thickener containing a carboxylic-acid-containing acrylic
polymer (C) [supplied by SAN NOPCO LIMITED under the trade name
"SN-Thickener 630", active component: 30 percent by weight] with water so
as to have an active component content of 5 percent by weight. The column
"[Number of moles of carboxyl-reactive functional group]/[number of moles
of carboxyl group]" in Table 1 shows the ratio of total number of moles
of carboxyl-reactive functional groups in total water-insoluble
crosslinking agents to the total number of moles of carboxyl groups in
total carboxyl-containing unsaturated monomers used as constitutive
monomers for the acrylic emulsion polymer (A).

Formation of Pressure-Sensitive Adhesive Layer and Production of
Pressure-Sensitive Adhesive Sheet

[0122] Next, the above-prepared, removable water-dispersible acrylic.
pressure-sensitive adhesive composition was applied to a corona-treated
surface of a PET film. (supplied by Mitsubishi Plastics, Inc. under the
trade name "T100N38", thickness: 38 μm) so as to have a dry thickness
of 15 μm, using an applicator supplied by Tester Sangyo Co., Ltd. The
article was dried in an oven with internal air (hot air) circulation at
128° C. for 2 minutes, aged at 50° C. for 3 days, and
thereby yielded a pressure-sensitive adhesive sheet.

Examples 2 to 8

[0123] A series of removable water-dispersible acrylic pressure-sensitive
adhesive compositions and a series of pressure-sensitive adhesive sheets
were prepared by the procedure of Example 1, except for using a different
type of a thickener containing a carboxylic-acid-containing acrylic
polymer (C) as given in Table 1.

Comparative Example 1

[0124] A removable water-dispersible acrylic pressure-sensitive adhesive
composition and a pressure-sensitive adhesive sheet were prepared by the
procedure of Example 1, except for not using the thickener containing a
carboxylic-acid-containing acrylic, polymer (aqueous solution of the
thickener containing a carboxylic-acid-containing acrylic polymer (C)),
as shown in Table 1.

Comparative Example 2

[0125] A removable water-dispersible acrylic pressure-sensitive adhesive
composition and a pressure-sensitive adhesive sheet were prepared by the
procedure of Example 1, except for using, instead of the aqueous solution
of the thickener containing a carboxylic-acid-containing acrylic polymer
(C), an aqueous solution prepared by diluting a urethane-associative
thickener [supplied by ADEKA CORPORATION under the trade name "ADEKA NOL
NH-550", active component 30 percent by weight] with water so as to have
an active component content of 5 percent by weight, as shown in Table 1.

Comparative Example 3

[0126] A removable water-dispersible acrylic pressure-sensitive adhesive
composition and a pressure-sensitive adhesive sheet were prepared by the
procedure of Example 1, except for not using the thickener containing a
carboxylic acid-containing acrylic polymer (C) and for using a
water-soluble crosslinking agent alone without using a water-insoluble
crosslinking agent.

Evaluation

[0127] Evaluations were performed on the pressure-sensitive adhesive
sheets prepared according to the examples and comparative examples by
measuring methods or evaluation methods as mentioned below. The
solvent-insoluble content of the pressure-sensitive adhesive layer (after
crosslinking) was measured by the aforementioned measuring method.

[0129] Each of the pressure-sensitive adhesive sheets prepared according
to the examples and comparative examples (sample size: 25 mm in width and
100 mm in length) was affixed to a deflector plate at a pressure of 0.25
MPa at a rate of 0.3 m/min using a laminator (small-sized laminator,
supplied by Tester Sangyo Co., Ltd.). The deflector plate was made from a
material triacetylcellulose (TAP) and had an arithmetic mean surface
roughness Re of about 21 nm in the machine direction (MD), about 31 nm in
the transverse direction (TD), and about 26 nm on an average in the
machine direction and the transverse direction.

[0130] The laminated sample of the pressure-sensitive adhesive sheet and
the deflector plate was left stand at an ambient temperature of
23° C. and relative humidity of 50% for 20 minutes, subjected to a
180-degree peel test under conditions mentioned below, an adhesive
strength (N/25 mm) of the pressure-sensitive adhesive sheet to the
deflector plate was measured, and this was defined as "initial adhesive
strength."

Adhesive Strength After Affixation and Storage at 40° C. for One
Week

[0131] Each of the pressure-sensitive adhesive sheets prepared according
to the examples and comparative examples (sample size: 25 mm in width and
100 mm in length) was affixed to a deflector plate at a pressure of 0.25
MPa at a rate of 0,3 m/min using a laminator (small-sized laminator,
supplied by Tester Sangyo Co., Ltd.). The deflector plate was made from a
material triacetylcellulose (TAC) and had an arithmetic mean surface
roughness Ra of about 21 nm in the machine direction (MD), about 31 nm in
the transverse direction (TD), and about 26 nm on an average in the
machine direction and the transverse direction.

[0132] The laminated sample of the pressure-sensitive adhesive sheet and
the deflector plate was stored at an ambient temperature of 40° C.
for one week, left stand at an ambient temperature of 23° C. and
relative humidity of 50% for 2 hours, subjected to a 180-degree peel test
under conditions mentioned below, an adhesive strength (N/25 mm) of the
pressure-sensitive adhesive sheet to the deflector plate was measured,
and this was defined as "adhesive strength after affixation and storage
at 40° C. for one week."

[0133] The 100-degree peel test was performed at an ambient temperature of
23° C. and relative humidity of 50% and at a tensile speed of 0.3
m/min using a tensile tester.

[0134] A sample can be evaluated as excelling in prevention of increase in
adhesive strength when having a difference between the initial adhesive
strength and the adhesive strength after affixation and storage at
40° C. for one week [(adhesive strength after affixation and
storage at 40° C. for one week)-(initial adhesive strength)] of
0.10 N/25 mm or less.

[0135] Each of the pressure-sensitive adhesive sheets prepared according
to the examples and comparative examples (sample size: 25 mm in width and
100 mm in length) was affixed to a deflector plate (supplied by Nitto
Denko Corporation under the trade name "SEG1425DUHC", 70 mm in width and
120 mm in length) at a pressure of 0.25 MPa at a rate of 0.3 m/min using
a laminator (small-sized laminator, supplied by Tester Sangyo Co., Ltd.).

[0136] The deflector plate affixed to the pressure-sensitive adhesive
sheet was as intact, left stand at 80° C. for 4 hours, and then
the pressure-sensitive adhesive sheet was removed therefrom. The
deflector plate, from which the pressure-sensitive adhesive sheet had
been removed, was left stand in a high-humidity environment (at a
temperature of 23° C. and relative humidity of 90%) for 12 hours,
the surface of which was visually observed, and less-staining properties
were evaluated according to the following criteria:

[0137] Good less-staining properties (Good): No change was observed in a
portion where the pressure-sensitive adhesive sheet had been affixed and
in a portion where the pressure-sensitive adhesive sheet had not been
affixed;

[0138] Poor less-staining properties (Poor): Whitening was observed in a
portion where the pressure-sensitive adhesive sheet had been affixed.

(3) Appearance (Presence or Absence of Dimples and Bubbles

[0139] The surface of the pressure-sensitive adhesive layer of each of the
pressure-sensitive adhesive sheets prepared according to the examples and
comparative examples was visually observed. The number of defects
(dimples and bubbles) in an observation area of 10 cm wide and 10 cm long
was measured, and the appearance was evaluated according to the following
criteria:

[0140] Good appearance (Good): The number of defects was from 0 to 100;

[0142] Each of the removable water-dispersible acrylic pressure-sensitive
adhesive compositions prepared in the examples and comparative examples
was applied to a surface, which. had been treated with a silicone, of a
PET film (supplied. by Mitsubishi Plastics, Inc. under the trade name
"MRF38") so as to have a dry thickness of 50 μm, dried in an oven
with, internal air circulation at 120° C. for 2 minutes, aged at
50° C. for 3 days, and thereby yielded a pressure-sensitive
adhesive layer.

[0144] The elongation at rupture (elongation at break) of the
pressure-sensitive adhesive layer sample was measured by performing a
tensile test at a tensile speed of 50 mm/min using a tensile tester,
while setting chucks so as to give an initial measurement length
(chuck-to-chuck distance) of 10 mm.

[0145] The elongation at rupture (elongation at break) indicates an
elongation at the time when the specimen (pressure-sensitive adhesive
layer sample) breaks, and is calculated according to the following
equation:

[0146] The abbreviations used in Table 1 are as follows. In Table 1, the
blending quantities of respective components of the removable
water-dispersible acrylic pressure-sensitive adhesive compositions are
indicated as actual blending quantities (blending quantities of the
commercial products).

[0162] Results in Table 1 demonstrates as follows. The pressure-sensitive
adhesive sheets (Examples 1 to 8) each having a pressure-sensitive
adhesive layer formed from a pressure-sensitive adhesive composition
satisfying the conditions specified in the present invention were
pressure-sensitive adhesive sheets having good appearance, not causing
whitening staining of deflector plates in a high-humidity environment
after affixation and removal of the pressure-sensitive adhesive sheets,
and thereby excelling in less-staining properties. These
pressure-sensitive adhesive sheets showed less increase in adhesive
strength with time after affixation.

[0163] In contrast, the samples containing no carboxylic-acid-containing
acrylic polymer (C) (Comparative Examples 1 and 2) suffered from crawling
defects in the pressure-sensitive adhesive layer and had poor
appearances. The sample using no water-insoluble crosslinking agent
(Comparative Example 3) had a high adhesive strength after affixation and
storage (adhesive strength after affixation and storage at 40° C.
for one week), thus showed a large increase in adhesive strength with
time, and caused staining.

[0164] As is described above, samples using pressure-sensitive adhesive
compositions not satisfying conditions specified in the present invention
failed to strike a balance between, appearance characteristics and
prevention of increase in adhesive strength.

[0165] While preferred embodiments of the present invention have been
described using specific terms, such description is for illustrated
purposes only, and it is to be understood that various changes and
modifications may be made without departing from the spirit, and scope of
the present invention as defined in the appended claims.